C. N. Afonso

Spanish National Research Council, Madrid, Madrid, Spain

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Publications (304)627.37 Total impact

  • R.J. Peláez · C.E. Rodriguez · C.N. Afonso · M. Škereň
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    ABSTRACT: This work shows that surface treatment of multilayer Au and Ag thin films with single UV nanosecond laser pulses is a versatile and an easy-to-use tool for tuning their optical response or colour in a wide spectral range. As a laser parameter, we have varied the fluence. As film parameters, we have studied the film configuration by changing the deposition order, number of layers as well as the composition by varying the thickness of the Au layer while keeping that of Ag constant. The laser exposure produces melting of the layers followed by mixing of the two metals and dewetting, which lead to nanoparticle (NP) formation. The optical response is dominated by the surface plasmon resonance (SPR) associated to the NPs whose wavelength, intensity and bandwidth are consistent with the observed NP morphologies, the number density and the variation of optical constants with Au content. It is dominated by the balance between the interband transition onset of the alloy and the SPR of the nanoalloyed NPs for low fluences and by multipolar interactions for high fluences. While the Au content allows tuning the SPR in the range of ≈ 180 nm, the laser fluence allows a fine tuning of the SPR features and the film configuration has little impact on the optical response. All together this allows tuning the colour in ranges broader than 20 CIELAB units along the two chromatic axes.
    No preview · Article · Nov 2015 · Surface and Coatings Technology
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    ABSTRACT: Ordering neural cells is of interest for the development of neural interfaces. The aim of this work is to demonstrate an easy-to-use, versatile, and cost/time effective laser-based approach for producing platforms that promote oriented neural growth. We use laser interferometry to generate fringed channels with topography on partially reduced graphene oxide layers as a proof-of-concept substrate. We study cell adhesion, morphology, viability, and differentiation in cultures of embryonic neural progenitor cells on platforms with a 9.4 μm period. Results evidence that fringed platforms significantly promote neurite alignment (≈50% at 6 d), while preserving viability and neural differentiation.
    No preview · Article · Oct 2015 · Macromolecular Bioscience

  • No preview · Article · Oct 2015 · ECS Transactions
  • R.J. Peláez · C.N. Afonso · M. Škereň · J. Bulíř
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    ABSTRACT: In this work, we compare patterns produced in Ag layers having similar thickness in the range 8.3–10.8 nm but having different initial nanostructure, i.e. behaving either as discontinuous or continuous layers and thus having very different thermal conductivities. The patterns are produced by exposing a phase mask to an excimer laser operating at 193 nm and using a projection optics that leads to similar fringed patterns with periods in the range 6.3–6.7 μm. The layer breaks up into isolated NPs due to laser induced melting at the regions around the intensity maxima sites. The resulting fringes have sharp interfaces in the case of discontinuous layers while a variety of regions across the pattern with no sharp interfaces are produced in the case of continuous layers. The results show that while the temperature distribution across the pattern matches almost perfectly the laser beam intensity profile for the former case, it becomes smeared due to lateral heat flow for the latter case. These results provide evidences for significant heating at the intensity minima sites that lead to solid-state dewetting and will eventually limit the minimum period achievable in the case of continuous metal layers or thermally conducting layers.
    No preview · Article · Sep 2015 · Applied Surface Science
  • R J Peláez · C N Afonso · M Škereň · J Bulíř
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    ABSTRACT: Fringed periodic patterns have been produced by laser interference at 193 nm in an almost continuous 9.5 nm-thick Ag film that exhibits a number density of ≈189 μm(-2) holes. Patterns with four periods in the range of 1.8-10.2 μm were produced by changing the projection optics. At high fluences, the film breaks up into nanostructures around the regions exposed to intensity maxima due to laser-induced melting. At low fluences, a new process is observed that is triggered at the initial holes of the film by solid-state dewetting. Once the fluence is high enough to prevent the temperature balance across the pattern, mass transport from cold to hot regions is observed, leading to film densification in regions around intensity maxima sites. The novel patterns are thus formed by fringes of material that is more/less dense than the as-grown film, each of which is located at intensity maxima/minima sites, and have negligible topography. Comparing the present results to earlier reports in the literature shows that the thermal gradient across the pattern is influenced by the initial film microstructure, rather than by the thickness. The existence of a minimum period, which is achievable depending on the thermal continuity of the film, is also discussed.
    No preview · Article · Jun 2015 · Nanotechnology
  • R. J. Peláez · T. Kuhn · C. E. Rodríguez · C. N. Afonso
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    ABSTRACT: Discontinuous metal films are converted into either almost round, isolated, and randomly distributed nanoparticles (NPs) or fringed patterns of alternate non transformed film and NPs by exposure to single pulses (20 ns pulse duration and 193 nm wavelength) of homogeneous or modulated laser beam intensity. The dynamics of NPs and pattern formation is studied by measuring in real time the transmission and reflectivity of the sample upon homogeneous beam exposure and the intensity of the diffraction orders 0 and 1 in transmission configuration upon modulated beam exposure. The results show that laser irradiation induces melting of the metal either completely or at regions around intensity maxima sites for homogeneous and modulated beam exposure, respectively, within ≤10 ns. The aggregation and/or coalescence of the initially irregular metal nanostructures is triggered upon melting and continues after solidification (estimated to occur at ≤80 ns) for more than 1 μs. The present results demonstrate that real time transmission rather than reflectivity measurements is a valuable and easy-to-use tool for following the dynamics of NPs and pattern formation. They provide insights on the heat-driven processes occurring both in liquid and solid phases and allow controlling in-situ the process through the fluence. They also evidence that there is negligible lateral heat release in discontinuous films upon laser irradiation.
    No preview · Article · Feb 2015 · Applied Physics Letters
  • R J Peláez · C N Afonso · M Skereň · J Bulíř
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    ABSTRACT: Periodic fringed patterns with four periods in the range 1.8-10.2 μm have been produced in continuous Ag films that have thicknesses of 14.6 nm and 19.5 nm by exposing a phase mask to single pulses of an excimer laser operating at 193 nm. The films were patterned either as-grown or after homogeneous exposure to the same laser beam. For fluences above the threshold, the films undergo liquid-state dewetting that, from low to high fluences, leads to their break into holes, fingers or elongated features and finally to isolated nanoparticles irrespective of the period, thickness or fluence. The period determines the range of fluences to achieve the different morphologies since the temperature profile across the pattern depends on the period due to the existence of significant lateral heat flow across the pattern. The maximum temperature achieved at the intensity maxima/minima sites thus decreases/increases as the period decreases, leading to solid-state dewetting at regions around the intensity minima; the shorter the period, the higher this type of dewetting. These regions eventually overcome the melting temperature for the shortest period and intermediate fluence, leading to the complete transformation of the films. Finally, the initial film morphology (discontinuities or holes) rather than thickness plays an essential role in the level of transformation at fluences around the threshold.
    No preview · Article · Jan 2015 · Nanotechnology
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    ABSTRACT: UV nanosecond laser pulses have been used to produce a unique surface nanostructuration of Ag@ZnO supported nanorods (NRs). The NRs were fabricated by plasma enhanced chemical vapor deposition (PECVD) at low temperature applying a silver layer as promoter. The irradiation of these structures with single nanosecond pulses of an ArF laser produces the melting and reshaping of the end of the NRs that aggregate in the form of bundles terminated by melted ZnO spherical particles. Well defined silver nanoparticles (NPs), formed by phase separation at the surface of these melted ZnO particles, give rise to a broad plasmonic response consistent with their anisotropic shape. Surface enhanced Raman scattering (SERS) in the as-prepared Ag@ZnO NRs arrays was proved by using a Rhodamine 6G (Rh6G) chromophore as standard analyte. The surface modifications induced by laser treatment improve the stability of this system as SERS substrate while preserving its activity.
    Full-text · Article · Jan 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: Ag and Au discontinuous films were exposed to single nanosecond pulses of a homogenized beam of an excimer laser operating at 193 nm. For low fluences, the films convert into big, almost spherical and isolated nanoparticles (NPs) due to laser-induced dewetting. Their optical response exhibits a sharp surface plasmon resonance (SPR) consistent with that of spherical and non-interacting NPs. For higher fluences, the formation of many small NPs and almost no big NPs is observed instead. The SPR features change and the plasmonic response becomes influenced by multipolar interactions among neighbouring NPs. Low and high fluence regimes are respectively related to melting and boiling threshold of the metal, and additionally, craters appear in the latter regime.
    Full-text · Article · Oct 2014 · Applied Surface Science
  • Ramon J. Pelaez · Timo Kuhn · Fidel Vega · Carmen N. Afonso
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    ABSTRACT: Patterns are fabricated on 290 nm thick nanostructured porous silicon layers by phase-mask laser interference using single pulses of an excimer laser (193 nm, 20 ns pulse duration). The dynamics of pattern formation is studied by measuring in real time the intensity of the diffraction orders 0 and 1 at 633 nm. The results show that a transient pattern is formed upon melting at intensity maxima sites within a time < 30 ns leading to a permanent pattern in a time < 100 ns upon solidification at these sites. This fast process is compared to the longer one (> 1 mu s) upon melting induced by homogeneous beam exposure and related to the different scenario for releasing the heat from hot regions. The diffraction efficiency of the pattern is finally controlled by a combination of laser fluence and initial thickness of the nanostructured porous silicon layer and the present results open perspectives on heat release management upon laser exposure as well as have potential for alternative routes for switching applications. (C) 2014 AIP Publishing LLC.
    No preview · Article · Oct 2014 · Applied Physics Letters
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    ABSTRACT: This work reports on the fabrication of 1D fringed patterns on nanostructured porous silicon (nanoPS) layers (563, 372, and 290 nm thick). The patterns are fabricated by phase-mask laser interference using single pulses of an UV excimer laser (193 nm, 20 ns pulse duration). The method is a single-step and flexible approach to produce a large variety of patterns formed by alternate regions of almost untransformed nanoPS and regions where its surface has melted and transformed into Si nanoparticles (NPs). The role of laser fluence (5–80 mJ cm−2), and pattern period (6.3–16 μm) on pattern features and surface structuring are discussed. The results show that the diameter of Si NPs increases with fluence up to a saturation value of 75 nm for a fluence ≈40 mJ cm−2. In addition, the percentage of transformed to non-transformed region normalized to the pattern period follows similar fluence dependence regardless the period and thus becomes an excellent control parameter. This dependence is fitted within a thermal model that allows for predicting the in-depth profile of the pattern. The model assumes that transformation occurs whenever the laser-induced temperature increase reaches the melting temperature of nanoPS that has been found to be 0.7 of that of crystalline silicon for a porosity of around 79%. The role of thermal gradients across the pattern is discussed in the light of the experimental results and the calculated temperature profiles, and shows that the contribution of lateral thermal flow to melting is not significant for pattern periods ≥6.3 μm.
    Full-text · Article · May 2014 · Journal of Applied Physics
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    Vincenzo Resta · Ramón J. Peláez · Carmen N. Afonso
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    ABSTRACT: This work studies the changes in the optical response and morphological features of 6±1 nm diameter Au nanoparticles (NPs) when covered by a layer of a-Al2O3 by pulsed laser deposition (PLD). The laser fluence used for ablating the Al2O3 target is varied in order to modify the kinetic energy (KE) of the species bombarding the NPs during their coverage. When the ion KE < 200 eV, the structural features and optical properties of the NPs are close to those of uncovered ones. Otherwise, a shift to the blue and a strong damping of the surface plasmon resonance is observed as fluence is increased. There are two processes responsible for these changes both related to aluminum ions arriving to the substrate during the coverage process, i.e. sputtering of the metal and implantation of aluminum species in the metal. Both processes have been simulated using standard models for ion bombardment, the calculated effective implanted depths allow explaining the observed changes in the optical response, and the use of a size-dependent sputtering coefficient for the Au NPs predicts the experimental sputtering fractions. In spite of the work is based on PLD, the concepts investigated and conclusions can straightforwardly be extrapolated to other physical vapor deposition techniques or processes involving ion bombardment of metal NPs by ions having KE > 200 eV.
    Full-text · Article · Mar 2014 · Journal of Applied Physics
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    ABSTRACT: Micromanipulation of dielectric objects, from polystyrene spheres to living cells, is achieved when radiation pressure forces create stable trapping by highly focused laser beams through microscopes. However, the impressive history of optical trapping is shadowed by the light diffraction limit, as research currently has focused on materials below the micron scale, requiring stronger optical confinement and higher intensities than can be provided by the conventional optical tweezers. Recently, plasmonic nanostructures have entered the field, either to assist or enhance it. In this study, we present experimental results on using localized fields of metallic structures for efficient trapping, with various patterns (dots, fringes and squares). The patterns were produced by laser interferometry on almost continuous Ag or Au films on glass and glass covered by an amorphous Al2O3 layer (10 nmthick) respectively.We have calculated the optical forces by measuring the particle’s escape velocity. The results show that the effective quality factor Q in the patterned metal film is enhanced by a factor >10, with respect to the unpatterned metal film and a factor >100, with respect to an uncoated glass. In addition, mathematical simulation of plasmonic fields is investigated to confirm and explain theoretically, the experimentally observed plasmonic enhancement.
    No preview · Article · Jan 2014
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    Dataset: A15 TSF

    Full-text · Dataset · Sep 2013
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    V. Resta · C. N. Afonso · E. Piscopiello · G. van Tendeloo
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    ABSTRACT: Metal nanoparticles (NPs) are often used as building blocks for nanostructured thin film systems, the properties of the system depending on the morphology and dimensions of the NPs. When using thin film technologies, the surface where the NPs nucleate on plays an important role on the growth process and eventually on the NPs features. The aim of this work is to produce metal NPs on substrates with different shapes. Gold NPs are produced by pulsed laser deposition on amorphous carbon, glass and MgO substrates at room temperature. The average dimensions of the NPs are controlled through the deposition time to have dimensions < 10 nm. The morphology of the NPs is studied by high resolution electron transmission microscopy (HRTEM) and the optical properties are studied by absorption spectroscopy. The results show that Au NPs are crystalline in all cases and for large NPs (> 5 nm), they are elongated due to coalescence and coarsening. The main difference when using amorphous or single crystalline substrate is that the many NPs become faceted and textured in the latter case. The optical spectra show however no significant differences. The results will be discussed in terms of the random orientation of the NPs across the substrate plane.
    Full-text · Dataset · Aug 2013
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    ABSTRACT: Pairs of samples containing Ag nanoparticles (NPs) of different dimensions have been produced under the same conditions but on different substrates, namely standard glass slides and a thin layer of amorphous aluminum oxide (a-Al2O3) on-glass. Upon storage in ambient conditions (air and room temperature) the color of samples changed and a blue-shift and damping of the surface plasmon resonance was observed. The changes are weaker for the samples on-glass and tend to saturate after 12 months. In contrast, the changes for the samples on a-Al2O3 appear to be still progressing after 25 months. While x-ray photoelectron spectroscopy shows a slight sulfurization and negligible oxidation of the Ag for the on-glass samples upon 25 months aging, it shows that Ag is strongly oxidized for the on a-Al2O3 samples and sulfurization is negligible. Both optical and chemical results are consistent with the production of a shell at the expense of a reduction of the metal core dimensions, the latter being responsible for the blue-shift and related to the small (<10 nm initial diameter) of the NPs. The enhanced reactivity of the Ag NPs on the a-Al2O3 supports goes along with specific morphological changes of the Ag NPs and the observation of nitrogen.
    No preview · Article · Aug 2013 · Nanotechnology
  • R. J. Peláez · C. N. Afonso · M. Bator · T. Lippert
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    ABSTRACT: The aim of this work is to demonstrate that single-photon photoionization processes make a significant difference in the expansion and temperature of the plasma produced by laser ablation of ceramic Al2O3 in vacuum as well as to show their consequences in the kinetic energy distribution of the species that eventually will impact on the film properties produced by pulsed laser deposition. This work compares results obtained by mass spectrometry and optical spectroscopy on the composition and features of the plasma produced by laser ablation at 193 nm and 248 nm, i.e., photon energies that are, respectively, above and below the ionization potential of Al, and for fluences between threshold for visible plasma and up to ≈2 times higher. The results show that the ionic composition and excitation of the plasma as well as the ion kinetic energies are much higher at 193 nm than at 248 nm and, in the latter case, the population of excited ions is even negligible. The comparison of Maxwell-Boltzmann temperature, electron temperatures, and densities of the plasmas produced with the two laser wavelengths suggests that the expansion of the plasma produced at 248 nm is dominated by a single population. Instead, the one produced at 193 nm is consistent with the existence of two populations of cold and hot species, the latter associated to Al+ ions that travel at the forefront and produced by single photon ionization as well as Al neutrals and double ionized ions produced by electron-ion impact. The results also show that the most energetic Al neutrals in the plasma produced at the two studied wavelengths are in the ground state.
    No preview · Article · Jun 2013 · Journal of Applied Physics
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    ABSTRACT: The aim of this work is to produce 2D plasmonic and diffractive structures in Ag films with sharp features for which both a deeper understanding of laser induced transformation upon modulated laser intensity and a correlation between structural and optical properties are required. We compare results obtained by exposing silver films to an excimer laser operating at 193 nm whose intensity is either modulated or homogeneous. In all cases, one laser exposure is enough to break the film into nanoparticles (NPs). The use of the modulated beam intensity leads to diffractive 2D patterns that are formed by rectangular regions of untransformed material surrounded by transformed regions covered by NPs. The former have sharp edges that are consistent with the absence of significant mass transport that is discussed in terms of the thermal gradient induced. The latter contain NPs whose diameter increases as the initial film effective thickness increases. The surface plasmons associated with the NPs in the transformed regions dominate the reflectivity spectrum and the 2D array formed by the untransformed regions is responsible for the diffractive properties. Evidence for spinodal dewetting is only observed in our case for the steep gradient conditions achieved at the border of the homogeneously irradiated regions.
    Full-text · Article · Mar 2013 · Nanotechnology
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    Full-text · Dataset · Jan 2013
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    Full-text · Dataset · Jan 2013

Publication Stats

4k Citations
627.37 Total Impact Points


  • 1988-2015
    • Spanish National Research Council
      • Institute of Optics "Daza de Valdés"
      Madrid, Madrid, Spain
  • 2009
    • ENEA
      Roma, Latium, Italy
  • 2001
    • Harvard University
      • School of Engineering and Applied Sciences
      Cambridge, Massachusetts, United States
  • 1996
    • University of Sussex
      Brighton, England, United Kingdom
    • King Juan Carlos University
      • Physics
      Мостолес, Madrid, Spain